We propose to continue studies designed to clarify the complex regulation of functions in E. coli that are induced in response to DNA damage. One aspect of the study concerns a strain (SC30) which originated as a recombinant betweena K12 tif-1 donor and a B/r recipient, crossed by conjugation. SC30 has a phenotype like tif-1, except that SOS functions are expressed constitutively at all temperatures. This constitutive expression is lexA plus-dependent, suggesting that it is not due to operator-constitutivity at the recA locus. The phenotype transduces with recA-flanking markers srl and alaS, and we believe it likely to be an alteration of the tif-1 allele affecting structural or regulatory sequences. We propose to clone the recA region of SC30 on a plasmid, and to analyze restriction fragments of the cloned region for comparison with analogous fragments of recA plus-DNA. Another aspect of our prosed work deals with the role of single-strand binding protein (SSB) in SOS regulation. We expect to quantify the amount of SSB in different strains in which we will also measure levels of recA protein. These studies permit a test of the hypothesis that competition for single-stranded regions of the DNA between these two binding proteins is the basis of the observed defect in SOS induction in strains having mutant SSB proteins. We are also continuing work designed to clarify the role of recA and lexA proteins in the regulation of normal cell division. Strains carrying both lon and frn (filamentous growth in rich medium) alleles have a conditionally lethal cell division defect in the presence of amino acid that is mitigated but not completely suppressed by recA or lexA mutations. We plan to test the hypothesis that these genes regulate an inhibitor of cell division that must be proteolytically cleaved in order for septation to occur in normal cell division, as well as after DNA is damaged.